The present invention generally relates to a reliable document authentication system and, in particular, relates to a reliable document authentication system using a public key cryptosystem.
Throughout history one of the tasks undertaken by many people and organizations has been proving the authenticity of documents. The importance of actually proving the authenticity of a document can range from merely identifying a signature to verifying military and/or political intelligence. Further, as often as one tries to demonstrate the authenticity of a document, there is usually at least one party that attempts to forge a document. Hence, there has been, and probably will continue to be, an ongoing struggle to be able to reliably authenticate documents.
Over the years technological advances have brought new meaning to the word "document". Today, a document may be, for example, an electronically generated receipt from a banking machine or a digitized recording on an optical recording disk. For the purpose of this patent application, therefore, the word "document" should be interpreted to include any information placed on any medium including, but not limited to, magnetic disks, optical disks or paper.
Another similar task that has just as colorful a history as document authentication is the secure communication of information between two parties. Such secure communication of information commonly includes the use of encryption/decryption techniques. Similar to the forger referred to above, there is usually at least one party that is interested in either stealing the information being communicated that has been encrypted or supplying false information in an encrypted format so that the receiver thereof is disinformed or both. Hence, throughout history various encryption/decryption schemes have been developed that, at least for a time, were thought to be secure only to discover that the security had been compromised. Again, technological advances have considerably changed the field of cryptography. For example, with modern computers many cryptographic techniques can be broken in a relatively short period of time due, primarily, to the speed that computers perform mathematical operations.
One presently secure cryptographic technique is generally known as the public key cryptographic system. One particular form of such a system is fully described and discussed in the basic article entitled "A Method for Obtaining Digital Signatures and Public Key Cryptosystems" by R. L. Rivest, A. Shamir and L. Adelmann, Volume 21 #2, February 1978, Communications of ACM pages 120-126. This particular system is frequently referred to as the RSA public key cryptosystem.
Public key techniques, as pointed out in the article entitled "Public Key Cryptography" by John Smith, in the January 1983 edition of Byte Magazine, pages 189-218, usually include two different kinds of keys: encryption keys and decryption keys. These keys includes the properties that: (a) it is possible to compute a pair of keys including an encryption key and a decryption key; (b) such that, for each pair, the decryption key that is not the same as the encryption key; and (c) it is not feasible to compute the decryption key even from the knowledge of the encryption key. In addition, in such a cryptosystem, the encryption and decryption keys are functionally reversible, i.e. if one key is used to encrypt the other key can be used to decrypt whatever has been encrypted.
As known, the name "public key" is derived from the fact that each party's encryption key is available, i.e. public, to all parties subscribing to the particular public key network involved. Hence, as currently used, public key cryptographic systems are designed for the direct communication between any two subscribing parties, each party having an unpublished decryption key and a published encryption key.
The public key cryptographic system has also found use in providing accurate identification of the source of a document. As discussed on pages 217-218 of the Smith article, a sender can effectively sign a message by first encrypt the message, or an authenticating portion thereof, such as, for example, the name of the sender using the private decryption key of the sender and then encrypt the message with the public encryption key of the receiving party. This results in a message portion that only the sender could have created and only the receiver can read. Hence, two party communication can, so long as public key cryptographic systems are secure, be implemented in such a fashion that the authenticity of a document can be ensured.
Nonetheless, there remain many instances where there is a need, or desire, for a third party to authenticate a document relevant to, or communicated between, two other parties. One example of such a situation would exist if a first party were required, or simply desired, to prove, or demonstrate, the authenticity of a particular document to a second party. In such a situation, it could be most beneficial if a third party could provide a means for authenticating that document. One particular situation could exist where a dispute over the authenticity of a document arose between two parties and an impartial third party was selected to resolve the issue to the satisfaction of both parties. Such a situation might arise when, in accordance with an agreement between two parties, one of the parties was to maintain certain records such that the second party could review those records to ensure compliance with the agreement. In such a situation it would be most beneficial if a third party were available to demonstrate the accuracy/inaccuracy of the records to the auditing second party.
Another more widely known situation that is representative of third party authentication of a document is in the mail handling field wherein a mailer must prove to postal delivery service the authenticity of having paid for the postage for a particular mailing. Currently, the United States Postal Service (USPS) accepts the indicia on an envelope applied by a mailer as representing that the postage required for the delivery of that envelope has been paid. In fact, in many instances, the indicia is applied by, for example, a leased postage meter and the manufacturer of the postage meter ensures to the USPS that when that postage meter is actuated to print the indicia, the postage, or the monetary value of the postage, has been paid. At the present time, postage meters apply the indicia to an envelope via a mechanical printing means such as a drum having the indicia etched thereon or via the impressing of a platen upon an envelope. However, due primarily to technological advances, the wide spread use of contactless printing has made it desirable to utilize such techniques in a mail handling system. However, the use of contactless printing techniques, at the present time, can lead to inaccurate accounting unless secure techniques are provided.
One secure technique would be the use of common encryption techniques wherein a mailer would have a cryptographic key that would allow the mailer to encrypt information and place that information on the envelope. The USPS, for example, could then, by using an identical cryptographic key, decrypt the information on the envelope and ensure that the proper postage for the delivery thereof has been paid. One major drawback of such a system, of course, is that there exists many thousands of mailers and hence, the USPS would be required to maintain a very large data base of cryptographic keys to enable it to decrypt all the different encryption keys distributed to the various mailers.
Consequently, it would be highly desirable to provide a system for reliably authenticating documents in general and, in particular, to reliably authenticate postage information placed on a mailing document.